More than 500 people remained unaccounted for Sunday across Colorado after deadly, historic flooding left weary residents reeling amid the dark forecast of more rain to come.

"I don't know that she's even OK," Rob Clements told The Coloradoan about his mother, Libby Orr, 73, who he last spoke with Thursday. Clements, who lives in Dallas, saw a photo of his mother's Big Thompson Canyon home in ruins on a Denver TV station's website. "I presume she is. But her house, if not completely gone, fell into the river and is most of the way gone."

About 350 people are unaccounted for in Larimer County, according to the county's sheriff's office. In adjacent Boulder County, more than 170 people were unaccounted for but were not considered missing yet, though they had not contacted family members.

"The sad thing is there's nothing we can do," Larimer County sheriff spokesman John Schulz told the Coloradoan, referring to the numerous phone calls the department has received from people who haven't heard from loved ones. "It's just taking time. It's so frustrating to people because there's no information available."

Areas from Denver to the Wyoming border remained under the threat of additional rain Sunday, with flash flood watches and warnings posted. Airlifts were set to continue with helicopter crews expanding their searches east to include Longmont, Fort Collins and Weld County.

The evacuations come as another round of storms swept into the battered state Saturday.The flood zone has grown to cover portions of an area nearly the size of Connecticut.

"It is a sinking feeling when you realize that when some people call ... we are not going to be able to get to them," Boulder County Sheriff Joe Pelle said. "But we are making great progress."

Two fatalities were identified by the Boulder County coroner Saturday as Wesley Quinlan and Wiyanna Nelson, both 19. Authorities believe the couple died when they were swept away after driving into floodwaters and then leaving their vehicle.

A missing woman could become the fifth confirmed death in the state. Witnesses saw floodwaters from the Big Thompson River destroy her home in the Cedar Cove area, Schulz said.

"We're sure there are going to be additional homes that have been destroyed, but we won't know that for a while," Schulz said. "I expect that we're going to continue to receive reports of confirmed missing and confirmed fatalities throughout the next several days."

By Saturday night, 1,750 people and 300 pets had been evacuated from Boulder and Larimer County, according to National Guard Lt. James Goff.

Thousands of people have fled homes in an area that normally sees less than 2 inches of rain in all of September but has been deluged by more than 14 inches this week alone, the National Weather Service said.

"We don't expect quite the level of intensity we've seen the last few days, but the soil is saturated, so it won't take much to do damage," Scott Entrekin, a weather service meteorologist in Boulder, told USA TODAY.

Entrekin said the storms, which have brought flooding in some areas that "we've never seen before," could ease Sunday. "Hopefully after that we will start drying out," he said.